A photovoltaic (PV) cell is a semiconductor transducer for converting electromagnetic radiation, particularly radiation in the visible or near visible (especially the infrared) portion of the spectrum into electrical energy. PV cells can be used as sensors in cameras and the like to obtain an electrical signal or to measure the incident or ambient light. They also can be used in power arrays to generate electrical power. PV cells are typically used to power electrical equipment for which it has otherwise proved difficult or inconvenient to provide a conventional source of continual electrical energy.
An individual PV cell has a distinct spectrum of light to which it is responsive as determined by the material forming the cell. Individual PV cells generate only a relatively small amount of power, for example, 0.5 to 3.0 watts. Consequently, for most power generation applications, multiple PV cells are connected together in series into an array. When using an array, a shadow or obstruction can temporarily or permanently cover one or more of its cells. These shaded cells do not receive light and are unable to generate power. Electrically a PV cell may be considered to be a current source in parallel with a diode that is forward biased relative to the current source. Problems arise in a series-connected PV cell array when an individual cell ceases to generate electricity because the diode characteristics of the inactive PV cell cause that cell to appear as a reverse-biased diode (i.e., an infinite or large resistor) to the other cells. The inactive cell blocks current flow and a large voltage develops across the cell. Consequently, the inactive PV cell, instead of functioning as a current source, functions as a power sink that can consume much of the energy produced by the remaining active PV cells in the array. The power output of the array falls dramatically. Moreover, the power consumed because of the inactive cell is converted into heat, which can easily damage that cell, surrounding cells, or the surrounding components. Exposure to large voltages for extended periods of time can cause the PV cell to break down permanently, resulting in an irreparable cell failure. In this case, even if the cell is later illuminated, it will continue to function as a resistor.
For example, a solar cell panel for space applications may require delivery of 29 V to a battery or load, calling for 28 series GaAs cells. If one cell were to fail from shading, as much as 27 V could be supplied in reverse bias across the shaded cell. A GaAs cell typically breaks down destructively in back bias at approximately 7 V because current is forced through local defects near the grid lines creating hot spots.
To prevent these problems, arrays customarily include a forward-biased diode (i.e., a bypass diode) connected in parallel across sets of one or more of the cells that form the array. Then, when one or more cells are inactive, current will flow around the cells through the bypass diodes, thereby eliminating the loss of power and the generation of heat otherwise occurring with the forward-biased diode of the inactive cell. While bypass diodes have proven a useful addition to arrays, they have their own disadvantages. Having to provide these diodes in addition to the cells adds to the overall cost and manufacturing complexity of any array. Furthermore, the addition of the diodes adds to both the weight and size of the cell array, which can be a significant disadvantage in space applications. Arrays that provide meaningful amounts of power for space applications have thousands or tens of thousands of PV cells, so the mass of a bypass diode for each cell becomes significant for launch cost and pointing and tracking following deployment. With the present invention, we avoid the need for separate, additional bypass diodes by constructing them into the booster cell of a tandem pair.